Superparamagnetic nanoparticles and nanocomposites

ABSTRACT

The present invention is directed to the syntheses of superparamagnetic nanoparticles and the incorporation of the nanoparticles as the magnetic component to form a strongly magnetic nanocomposite. The superparamagnetic nanoparticles possess no hysteresis and are too small to support eddy currents. The invention uses a ligand exchange procedure to produce aminated nanoparticles that are then cross-linked using epoxy chemistry. The result is a magnetic nanoparticle component that is covalently linked and well separated. By using this ‘matrix-free’ approach, it is possible to substantially increase the magnetic nanoparticle fraction, while still maintaining good separation, leading to a superparamagnetic nanocomposite with strong magnetic properties and low magnetic losses.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Contract No.DE-NA0003525 awarded by the United States Department of Energy/NationalNuclear Security Administration. The Government has certain rights inthe invention.

FIELD OF THE INVENTION

The present invention relates to magnetic materials and, in particular,to a method to synthesize superparamagnetic nanoparticles that can beused to form a superparamagnetic nanocomposite.

BACKGROUND OF THE INVENTION

Recent developments in wide-bandgap power electronics have led tosignificant improvements in the power to size ratio. However, thepassive magnetic components have shown less significant changes inrecent years, and now represent a disproportionate amount of space andweight in the system. This has become a pressing issue for modernapplications, e.g., small sizes and high switching frequencies areneeded for notebook computers. See M. Koeda et al., Electr. Commun. Jpn.96, 46 (2013). Furthermore, reduction in power consumption and enhancingoverall efficiencies has become more imperative as the drive for alow-carbon economy continues. Rapid advancement in soft magneticmaterials for the next generation of power electronics is thereforesorely needed. Currently, carbonyl iron and associated ferrites are usedextensively as powder cores for inductor applications in high-powercircuits. However, these are characterized by losses from remanentmagnetization and eddy current formation, effects that are particularlyevident at high switching frequencies. More highly desired magneticproperties include high saturation magnetization and permeability, lowconductivity (to avoid eddy current losses), and low magnetichysteresis. Ultimately, the goal is to combine high magnetic saturation,M_(sat), with a nearly flat permeability response from DC up to severalMHz, performance superior in terms of permeability and loss behavior tothat offered by soft ferrites. See C. Beatrice et al., J. Magn. Magn.Mater. 420, 317 (2016).

All of these design criteria can be met by an appropriately designedsoft magnetic material, which avoids the common sources of loss.Superparamagnetic nanoparticles are a class of material that have seenintense research interest in fields including drug delivery, bimodalimaging, biosensing, and heavy metals recovery. See K. Mandel et al.,ACS Appl. Mater. Interfaces 4, 5633 (2012); W. J. Dong et al., Adv.Mater. 23, 5392 (2011); W. L. Gu et al., Anal. Chem. 87, 1876 (2015);and L. J. Zhu et al., J. Controlled Release 169, 228 (2013).Superparamagnetism is a phenomenon that occurs in single domainparticles, where the collective behavior of atomic spins leads to agiant vector spin that can randomly orient with sufficient thermalenergy, leading to a net zero magnetization for the particle ensemble.Superparamagnets are defined by an absence of magnetic hysteresis, whichmakes them especially suitable for high frequency switchingapplications. The size of the particle required for superparamagnetismto emerge is also relatively small, which eliminates the contributionfrom eddy current loss, as the nanoparticles themselves are too small tosupport eddy currents. Therefore, superparamagnetic nanoparticles, intheory, should completely remove the two major sources of loss whencompared to conventional core materials.

A strong candidate for effective application are iron nanoparticles.Iron is low-cost, being the fourth most abundant element, and isnon-toxic. It possesses the highest room temperature M_(sat) of anyelement (218 Am²/kg@293 K), while also possessing a very lowmagnetocrystalline anisotropy, meaning superparamagnetism can beobserved at larger nanoparticle sizes. See B. Cullity, Introduction toMagnetic Materials, Addison-Wesley Pub. Co., Reading, Massachusetts(1972). This is important when optimizing the material's M_(sat) asmagnetization reduces with decreasing nanoparticle size. This istypically due to the formation of magnetically inert layers at thesurface arising from spin-glass formation, or from surface electroniceffects. See D. L. Huber, Small 1, 482 (2005).

In order to take advantage of superparamagnetic iron nanoparticles incore applications, it is necessary to separate the particles and preventmagnetic interaction. For example, magnetic dipole-dipole interactionscan introduce hysteresis into a superparamagnetic ensemble. See M.Knobel et al., J. Nanosci. Nanotechnol. 8, 2836 (2008). Separation haspreviously been achieved by the formation of a polymer nanocomposite, inwhich the nanoparticles are suspended in a polymer matrix. See J. Pyun,Polymer Reviews 47, 231 (2007). Polymer nanocomposites have attractedsignificant research interest due to facile formation, light weight, andlow cost of the matrix fraction. Furthermore, the plethora of differentpolymer and nanoparticle species available allows for the design ofmaterials with tunable mechanical, magnetic, optical, and electricalproperties. See H. Wakayama and H. Yonekura, Mater. Lett. 171, 268(2016). This has led to a number of useful applications fornanocomposites including sensor applications, as conducting membranesfor fuel cells, and as fire retardants. See J. Pyun, Polymer Reviews 47,231 (2007). Polymeric nanocomposites can also be applied relativelyeasily to molds and also see promise in additive manufacturing. See A.C. de Leon et al., React. Funct. Polym. 103, 141 (2016). The realpromise of nanocomposites however lies in their multi-functionality andthe possibility of realizing unique combinations of propertiesunachievable with traditional, bulk materials. The inherent challengesin their formation include control over the distribution in size anddispersion of the nanostructured constituents, as well as tailoring andunderstanding the role of interfaces on the emerging bulk properties.Phase separation is also a well-established concern, which for amagnetic nanocomposite would eliminate the benefits ofsuperparamagnetism due to the formation of ferromagnetic domains. See J.B. Hooper and K. S. Schweizer, Macromolecules 39, 5133 (2006). By theirnature however, polymers have very large molecular weights, and aretypically benign in terms of functionality. This means that in ananocomposite the functional component becomes the minority fraction,which limits performance. Increasing this fraction to achieve highnanoparticle loadings, while still possessing control over theinterparticle spacing and magnetic interactions would significantlyincrease the performance and applicability of nanocomposites.

Recently, a supramolecular building block approach has been suggestedfor the preparation of a new family of nanocomposites. Thesenanocomposites are comprised of nanoparticles cross-linked by covalentlybound organic bridges, eliminating the need for a polymer matrix. See V.N. Mochalin et al., Acs Nano 5, 7494 (2011); and B. I. Dach et al.,Macromolecules 43, 6549 (2010). The nanoparticles are separated by thesurfactant molecules bound to their surfaces, which are covalently boundto neighboring nanoparticles through their corresponding surfactantmolecules. In doing so, a “matrix-free” nanocomposite is formed. Thesenanocomposites are not prone to the nanoparticle aggregation effectsthat plague conventional nanocomposites, and provide exceptionally highstrength and toughness. See V. N. Mochalin et al., Acs Nano 5, 7494(2011).

A promising approach to forming matrix-free nanocomposites is byemploying epoxy chemistries, as this is well-known to provide strongmechanical properties in a cross-linked environment. Epoxy resins are aclass of thermosetting polymers that are ubiquitous as coatings,adhesives, and in structural repair and are recently seeing applicationin additive manufacturing applications. See B. G. Compton and J. A.Lewis, Adv. Mater. 26, 5930 (2014); and F. L. Jin et al., J. Ind. Eng.Chem. 29, 1 (2015). They have also been used on numerous occasions toform traditional nanocomposite materials. Carbonyl iron-epoxy magneticcores have recently been used by Sugawa for large-current inductorsmounted directly onto a chip. See Y. Sugawa et al., IEEE T. Magn. 49,4172 (2013). They showed that good dispersion within the epoxy matrixleads to lower losses at high frequencies, due to less large magneticagglomerates present in the system. Gu surface functionalized magnetitenanoparticles with conductive polyaniline to increase theepoxy-nanoparticle interaction and strengthen the nanocompositemechanical properties. See H. Gu et al., ACS Appl. Mater. Interfaces 4,5613 (2012). Incorporation of the functionalized nanoparticles led tobetter thermal stability as well as increased dispersion of magneticfraction. Zhu formed magnetic epoxy nanocomposites with Fe@FeOcore-shell nanoparticles. See J. H. Zhu et al., ACS Appl. Mater. Inter.2, 2100 (2010). They used a commercially available epoxy system andformed nanocomposites with nanoparticle packing fractions of between 1and 20 wt. %. They measured an M_(sat) of 108 Am²/kg for the Fe@FeOnanoparticles themselves, which was reduced to 17 Am²/kg whenincorporated into the epoxy network. Pour also showed improvedmechanical properties when incorporating surface modified maghemiteα-Fe₂O₃ nanoparticles into a diglycidyl ether of bisphenol-A(DGEBA)-based epoxy matrix. See Z. S. Pour and M. Ghaemy, Prog. Org.Coat. 77, 1316 (2014). This was due to improved nanoparticle dispersion,and increased interfacial adhesion between the DGEBA and α-Fe₂O₃.However, maximum nanoparticle loading was only 11 wt %. While providinggood examples of the usefulness an epoxy network in the formation ofnanocomposites, these approaches mimicked the use of polymers in thatthe nanoparticles were embedded in an epoxy matrix.

In terms of the nanoparticle fraction, control over the size and shapeis essential to produce an effective superparamagnetic nanocomposite.For example, a finite size distribution leads to a distribution inrelaxation times, which can adversely affect performance in highfrequency switching applications. See B. T. Naughton et al., J. Am.Ceram. Soc. 90, 3547 (2007). When considering shape, any deviation froman ideal sphere can introduce higher-order multipole terms in themagnetic dipole interaction energy, leading to deviations from theexpected magnetic behavior. See N. Mikuszeit et al., J. Phys. Condens.Mat. 16, 9037 (2004). Controlling interparticle spacing is imperative;too close and interparticle interactions can lead to hysteresis andlosses, too far and porosity can reduce the maximum achievable magneticfraction; reducing the overall M_(sat) of the nanocomposite. See B. T.Naughton et al., J. Am. Ceram. Soc. 90, 3547 (2007). Finally, themagnetic nanoparticles employed in the formation of the nanocompositemust be synthesized in sufficiently large quantities. This is especiallyimportant when considering application of the nanocomposite in inductorand transformer technologies, where the form-factor for testing can varysignificantly.

SUMMARY OF THE INVENTION

The present invention is directed to the synthesis of superparamagneticnanoparticles in a predictable and reproducible manner. Thenanoparticles have high saturation magnetizations and aresuperparamagnetic at room temperature, thereby addressing the two majorrequirements for magnetic components in power electronics. Thenanoparticles can be subjected to a ligand exchange procedure in-situ,which replaces the growth directing surfactants with an amine ligandcomprising two or more amine groups. The resulting aminatednanoparticles can then react with an epoxy comprising two of moreepoxide groups, leading to a covalently bound cross-linked networkbetween the nanoparticle fraction. By using this matrix-free approachthe organic fraction of the nanocomposite can be significantly reduced.In one example, 62 wt % loading of Fe/Fe_(x)O_(y) core-shellnanoparticles was achieved. The nanocomposite is superparamagnetic atroom temperature and has a large saturation magnetization. The magneticfraction is well separated in the nanocomposite. The resultingsuperparamagnetic nanocomposite can therefore be used as a core materialfor inductor and transformer technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will refer to the following drawings, whereinlike elements are referred to by like numbers.

FIG. 1 is a schematic illustration of an as-synthesized matrix-freesuperparamagnetic nanocomposite. The interparticle distance can then betuned easily by varying the alkyl chain length of the diamine.

FIG. 2(a) is an ensemble small angle X-ray scattering (SAXS) analysisperformed on an aliquot of Fe/Fe_(x)O_(y) core-shell nanoparticlesmeasured an average diameter of 15.2 nm±1.2 nm. FIG. 2(b) shows acorresponding transmission electron microscopy (TEM) image. (inset)High-resolution TEM image showing partial surface oxidation has occurredduring the sample preparation step.

FIG. 3(a) is a high-resolution transmission electron microscopy (HRTEM)image of a Fe/Fe_(x)O_(y) core-shell nanoparticle. FIG. 3(b) is a highmagnification image of the iron oxide shell. The lattice spacings can beindexed to the (311) and (220) planes characteristic of either magnetite(Fe₃O₄) or maghemite (γ-Fe₂O₃).

FIGS. 4(a) and (b) show magnetic characterization of the 15.2 nm±1.2 nmFe/Fe_(x)O_(y) core-shell nanoparticles performed using a vibratingsample magnetometer (VSM). FIG. 4(a) is a magnetic moment vs field plot,showing the nanoparticles have a saturation magnetization, M_(sat), of96 Am²/kg at 50 K. The M_(sat) curve was fit using the Langevinequation, indicating the nanoparticles had a magnetic diameter of 10.2nm. FIG. 4(b) is a zero-field cooled-field cooled (ZFC-FC) plot showinga blocking temperature of 233 K, which indicates the nanoparticles aresuperparamagnetic at room temperature.

FIG. 5 is a graph of thermogravimetric analysis (TGA) and differentialscanning calorimetry (DSC) analysis of the cured nanocomposite formedusing 1,6-diaminohexane.

FIG. 6(a) is a plot of VSM magnetometry of the as-cured nanocomposite,compared to the as-synthesized Fe/Fe_(x)O_(y) core-shell nanoparticles.The nanocomposite has an M_(sat) of 58 Am²/kg, compared to 96 Am²/kg forthe nanoparticles. The initial mass susceptibility has decreased from1.0×10⁻³ m³/kg to 5.2×10⁻⁴ m³/kg. FIG. 6(b) is a graph showing M_(sat)of the magnetic fraction of the nanocomposite only, giving a reducedvalue of 76 Am²/kg. The M_(sat) curve was fit with the Langevin equationwhich indicates the magnetic diameter of the nanoparticles has reducedfrom 10.2 nm to 9.5 nm during the curing process.

FIG. 7 are zero-field cooled curves performed using VSM magnetometry.The Fe/Fe_(x)O_(y) nanoparticles show a broad peak with a blockingtemperature, T_(B), of 233 K indicating they are superparamagnetic atroom temperature. When cured into the superparamagnetic nanocomposite, areduction in T_(B) to 147 K, 142 K, and 146 K is observed fornanocomposites formed using 1,6-diaminohexane, 1,8-diaminooctane, and1,12-diaminododecane, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to the syntheses of stronglyparamagnetic nanoparticles and their use as the magnetic fraction in theformation of a matrix-free superparamagnetic nanocomposite. FIG. 1 showsa schematic illustration of the matrix-free superparamagneticnanocomposite. The nanoparticles can be synthesized using a reversibleagglomeration mechanism, which is scalable and yields a magneticnanoparticle ensemble with good shape control and tight sizedistribution. See D. L. Huber, U.S. Pat. No. 7,972,410 (2011), which isincorporated herein by reference. A ligand-exchange procedure can thenbe performed in-situ, replacing the growth directing surfactants with adiamine or higher order amine ligand, thereby expressing an aminefunctionality into the environment. This can then be cross-linked withan epoxy comprising at least two epoxide groups to yield a matrix-freenanocomposite. By covalently linking the magnetic nanoparticles togetherwith a cured epoxy network the organic fraction can be significantlyreduced to achieve high nanoparticle loadings. The nanocomposite has awell separated nanoparticle fraction, enabling a nanocomposite withstrong magnetic properties.

Synthesis of Superparamagnetic Nanoparticles

As described above, iron nanoparticles are a good candidate for thesuperparamagnetic nanocomposite, and iron has the highest roomtemperature saturation magnetization and a low magnetocrystallineanisotropy. However, other magnetic nanoparticles, such as cobalt,nickel, and alloys thereof, can also be used. As an example,Fe/Fe_(x)O_(y) core-shell nanoparticles can be formed via solvothermalsynthesis using trioctylphosphine (TOP) and oleylamine (OLA) assurfactants, with 1-octadecene (1-ODE) as the high boiling pointsolvent. Solvothermal synthesis is known to give significantly bettercontrol over the synthesis of iron nanoparticles when compared to othertechniques, e.g., reduction of iron salts. See D. L. Huber, Small 1, 482(2005). Magnetic nanoparticles with a uniformly spherical shape and avery tight size distribution can be formed reproducibly and predictablyin an Extended LaMer synthesis. See E. C. Vreeland et al., Chem. Mater.27, 6059 (2015). Here, continuous addition of the reaction precursorleads to a steady state growth regime which eliminates Ostwald ripening,a known source of polydispersity. This method was extended to stronglymagnetic zero-valent iron nanoparticles, in what has been named thereversible agglomeration mechanism. See D. L. Huber, U. S. Pat. No.7,972,410 (2011). In this approach, magnetic nanoparticles nucleate andgrow until a critical susceptibility is reached. At this point, magneticdipole interactions overcome dispersive forces leading to magneticagglomeration. This is followed by precipitation of the agglomeratednanoparticles and a phase change in the system, an example of truethermodynamic reaction control in nanoparticle synthesis. As the onsetof agglomeration is directly a result of an increase in particlesusceptibility, nanoparticle growth is arrested within a very small sizewindow, yielding a strongly magnetic nanoparticle ensemble with a tightsize distribution. With the continued addition of precursor a secondnucleation event can then occur, leading to subsequent growth and asecond agglomeration/precipitation event. By continuously addingprecursor, multiple reversible agglomeration steps can be performedindefinitely, meaning the mechanism is unique in its scalability. Thisis an important aspect of this synthetic approach, since for effectiveapplication of a useful nanocomposite, greater than typical lab-scalequantities need to be produced. The nanoparticle product can then beresuspended in most common organic solvents through the addition ofenergy, e.g., heat or sonication, confirming the reversibility of themechanism. In the synthesis of iron nanoparticles, solvents and reagentsare typically dried and degassed to remove oxygen and avoid oxidation.This is reasonable for research scale quantities, however when dealingwith the larger quantities required for scale up this can becomeimpractical. Therefore, the following nanoparticle products were allformed using the solvents and reagents as received directly from thesupplier.

As an example of the invention, Fe/Fe_(x)O_(y) nanoparticles weresynthesized using the reversible agglomeration mechanism. To form 1 gmof nanoparticles, a three-necked reaction flask was charged with 16 mL1-octadecene (1-ODE), 2 mL oleylamine (OLA) (6 mmol), and 2.7 mLtrioctylphosphine (TOP) (6 mmol). This was transferred to an air freeSchlenck line under a nitrogen atmosphere and heated to 250° C. withstirring at 300 rpm, using a magnetic stir bar. The reaction solutionwas prepared by diluting 5 mL iron pentacarbonyl (Fe(CO)₅) (37 mmol) in15 mL 1-ODE. The reaction solution was injected into the reaction flaskat a rate of 100 μL/min via a syringe pump. The reaction was reacted fora further 30 min at 250° C., following the completion of the drip. Thereaction was then cooled to room temperature and the product wasisolated from the magnetic stir bar and resuspended in 1-ODE. Thesurface was allowed to oxidize passively, leading to Fe/Fe_(x)O_(y)core-shell nanoparticles.

The results of the synthesis of superparamagnetic nanoparticles areshown in FIGS. 2(a)-(b). The Fe/Fe_(x)O_(y) core-shell nanoparticlessynthesized using the reversible agglomeration mechanism can beseparated from solution by the application of an external magneticfield. Ensemble small angle X-ray scattering (SAXS) size analysis isgiven in FIG. 2(a), showing the raw data overlaid with the model fit andcorresponding residuals. The model was applied assuming a sphericalshape and Gaussian size distribution, as observed in transmissionelectron microscopy (TEM), as shown in FIG. 2(b). SAXS gave an averagesize of 15.2 nm±1.2 nm, for the nanoparticle ensemble. TEM analysisshowed the nanoparticles to be uniformly spherical, with a 2.5 nm thickiron oxide shell which forms after the nanoparticle product is exposedto air post-synthesis (inset of FIG. 2(b)).

FIG. 3(a) is a high-resolution transmission electron microscopy (HRTEM)of a Fe/Fe_(x)O_(y) core-shell nanoparticle. FIG. 3(b) is a highmagnification image of the iron oxide shell. The lattice spacings can beindexed to the (311) and (220) planes of the cubic spinel phase,characteristic of either magnetite (Fe₃O₄) or maghemite (γ-Fe₂O₃).Unfortunately, TEM alone is not able to distinguish between the twophases.

The magnetic properties of the 15.2 nm±1.2 nm Fe/Fe_(x)O_(y) core-shellnanoparticles were then investigated using vibrating sample magnetometry(VSM). As shown in FIG. 4(a), saturation magnetization, M_(sat), of thenanoparticles was measured to be 96 Am²/kg at 50 K. This is larger thanthe value expected for magnetite (92 Am²/kg) or maghemite (76 Am²/kg),indicating the presence of a zero-valent iron (Fe(o)) core. The M_(sat)curve was then fit using the Langevin equation, as shown in FIG. 4(a).The fit gave a magnetic diameter of 10.2 nm, which is consistent withwhat is observed for the zero-valent iron core observed in TEM. See M.Unni et al., ACS Nano 11, 2284 (2017). HRTEM did not observe latticespacings of the iron core characteristic of the bcc crystal structure ofα-Fe, indicating the nanoparticles possess either an amorphous core, ora distorted lattice, both of which are known to reduce M_(sat) whencompared to α-Fe. See T. C. Monson et al., Part. Part. Syst. Charact.30, 258 (2013). An additional factor reducing the M_(sat) may be thestrong iron-phosphine interaction that can introduce a magnetically deadlayer at the surface. See D. L. Huber et al., Small 1, 482 (2005).Zero-field cooled, field cooled (ZFC-FC) experiments were performed toidentify the blocking temperature, T_(B), and the corresponding onset ofsuperparamagnetism, as shown in FIG. 4(b). The peak is relatively broadand centered with a T_(B) of 233 K, which is significantly reduced whencompared to zero-valent iron nanoparticles of a similar size, due to thepresence of an oxide shell. See J. Watt et al., Nanoscale 9, 6632(2017). Here, oxidation is beneficial as it ensures that thenanoparticles are superparamagnetic at room temperature. The broadnessof the peak indicates there may be some interparticle interactionsoccurring in the nanoparticle ensemble, which is a reasonable assumptionas the product is measured as an agglomerated magnetic pellet, as isproduced from the reversible agglomeration mechanism.

The reversible agglomeration mechanism is characterized by thenucleation and growth of nanoparticles followed by agglomeration andrenucleation steps, a cycle that can be repeated indefinitely to reachlarge scale synthesis. Therefore, to calculate the number of reversibleagglomeration cycles needed to synthesize 1 g of Fe/Fe_(x)O_(y)nanoparticles, the critical nuclei radius at the reaction temperature of523 K was first determined. The critical nuclei radius is defined by:

$r_{c} = {- \frac{2\gamma}{G_{V}}}$where γ is the surface free energy of iron, which at 523 K is equal to2.3 J/m². See J. Park et al., Angew. Chem. Int. Ed. Engl. 46, 4630(2007); S. Schonecker et al., Sci. Rep. 5, 14860 (2015); and G. Grocholaet al., J. Chem. Phys. 120, 3425 (2004). G_(v) is the Gibbs energy pervolume of iron, which in the molten state of Fe(0) nuclei at hightemperatures is equal to −1.57×10⁹ J/m³. See J. Park et al., Angew.Chem. Int. Ed. Engl. 46, 4630 (2007). In the reducing environmentcreated by the decomposition of Fe(CO)₅ and the production of CO gas,the nuclei can be assumed to be truly zero-valent iron. Therefore, thecritical nuclei radius for Fe(0) at 523 K is 2.93 nm. If it assumed thatthe first drop of precursor nucleates and all iron is consumed bysurviving nuclei, then how much additional precursor is needed to growthe nanoparticles to their maximum size, before agglomeration occurs,can be calculated. Each individual drop has a volume of ˜10 μL. See G.K. Tripp et al., Vet. Ophthalmol. 19, 38 (2016). Therefore, as a 3:1ODE:Fe(CO)₅ precursor mixture is introduced, each drop contains 2.5 μLof Fe(CO)₅, which equates to 1.06 mg of Fe(0). Taking a nuclei size of2.93 nm and a density of 7874 kg/m³, each drop contains 1.28×10¹⁵nuclei. Then, for the 1 g reaction, the maximum size beforeagglomeration is 15.2 nm. Each maximum-sized nanoparticle is thereforecalculated to have a mass of 1.45×10⁻²⁰ kg. If each of the 1.28×10¹⁵nuclei grows to this maximum size, then one reversible agglomerationcycle consumes 1.85×10⁻⁵ kg, or 0.0185 g, of Fe(0). Taking the aboveassumptions, and knowing that the final mass of Fe(0) nanoparticlesformed is 1 g, a minimum of 54 cycles of reversible magneticagglomeration is calculated to have occurred.

The reversible agglomeration mechanism was scaled up by an order ofmagnitude, to 10 g. At this large scale, a peristaltic pump was neededto deliver sufficient quantities of Fe(CO)₅ precursor solution to thereaction flask. Likewise, the size of the reaction flask dictates thatmagnetic stirring is not sufficient, and an overhead stirring setup wasrequired. This introduces challenges with controlling an air-freeenvironment, however this was achieved by maintaining a positivepressure of flowing nitrogen throughout the reaction. Specific Tygontubing was chosen with low gas permeability and high chemicalresistance, to protect against premature decomposition of the Fe(CO)₅.To form 10 gm of nanoparticles, a three-necked reaction flask wascharged with 80 mL 1-ODE, 10 mL OLA (30 mmol), and 13.6 mL TOP (30mmol). This was transferred to an air free Schlenck line under anitrogen atmosphere and heated to 240° C. with stirring at 300 rpm usingan overhead stirrer. The reaction solution was prepared by diluting 25mL Fe(CO)₅ (185 mmol) in 75 mL 1-ODE. The reaction solution was injectedinto the reaction flask at a rate of 0.33 mL/min using a peristalticpump. The reaction was reacted for a further for 30 min at 240° C.,following the completion of the drip. The reaction was then cooled toroom temperature and the product was isolated and resuspended in 1-ODE.The surface was allowed to oxidize passively, leading to Fe/Fe_(x)O_(y)core-shell nanoparticles.

SAXS analysis gave an average diameter of 13.7 nm±2.5 nm for thisnanoparticle ensemble. Again, the raw data was model fit assuming aspherical shape and Gaussian size distribution. TEM analysis showed aslight loss of size and shape control, which can be attributed tothermal and concentration gradients associated with the larger reactionvessel required to carry out the synthesis. It is also important to notehere that the slight deviation from a spherical shape will lead to anincrease in the calculated size distribution from SAXS analysis. See T.Li et al., Chem. Rev. (2016). The same calculations as above can be usedto determine the number of reversible agglomeration cycles needed tosynthesize 10 g of 13.7 nm nanoparticles. 1.36×10⁻⁵ kg of Fe(0) isrequired for each cycle, therefore the 10 g scale up reaction is theresult of a minimum of 735 cycles of reversible magnetic agglomeration.The number of cycles needed was increased by more than an order ofmagnitude due to the reduction in overall nanoparticle size. Despite theslight loss of size and shape control, the magnetic properties of theensemble are expected to be uniform as arresting of nanoparticle growthby magnetic agglomeration is defined by particle susceptibility, notshape.

Formation of Superparamagnetic Nanocomposite

As an example of the invention, the 15.2 nm±1.2 nm Fe/Fe₃O₄ core-shellnanoparticles formed in the 1 g reaction were used as the magneticfraction for the formation of a matrix-free superparamagneticnanocomposite. The nanoparticles were subjected to a ligand exchangeprocedure in-situ, replacing the monoamine OLA and TOP with analkyl-diamine ligand (for example, 1,6-diaminohexane). It is expectedthat the amine on the surface is replaced by adsorption dynamics,whereas the TOP has previously been shown to be replaced by ligands withamine functionality. See Y. Xu et al., Langmuir 27, 8990 (2011). Theligand exchange procedure means the nanoparticles express an aminefunctionality into the environment, which is known to be reactivetowards cross-linking epoxides. In addition to alkyl-diamines, higherorder amines, e.g., triamines, and aromatic amines comprising at leasttwo amine groups can also be used for ligand exchange. The aminatednanoparticles were then reacted with a triepoxy (for example,N,N-diglycidyl-4-glycidyloxyaniline) in chloroform. The solvent wassubsequently removed to give a viscous, workable liquid that can becured at elevated temperatures (e.g., 60° C.) to yield a cross-linkedepoxy network characterized by covalent linkages between the magneticnanoparticles, as shown in FIG. 1. Other epoxy compounds comprising atleast two epoxide groups can also be used to form the cross-linked epoxynetwork. By crosslinking directly to a species that is covalently boundto the nanoparticle surface, the amount of organic fraction can bereduced substantially when compared to polymeric matrices. This allowsfor a significant increase in the magnetic fraction, optimizing themagnetic properties of the resulting nanocomposite. This approach alsomeans that the interparticle spacing can be easily tuned, with thechoice of alkyl chain length directly determining the distance betweenparticles.

To confirm that the nanoparticles are undergoing a ligand exchangeprocedure at the surface, ex-situ experiments on the Fe/Fe_(x)O_(y)core-shell nanoparticles were performed to mimic the conditions foundduring nanocomposite formation. To do this an aliquot of purifiednanoparticles were reacted with a mixture of 1,6-diaminohexane inchloroform. The aminated nanoparticles were then washed thoroughly toremove any excess ligand not covalently bound to the surface of theparticles. A small quantity of fluorescamine was then introduced, whichis a spiro compound that forms highly fluorescent pyrrolinones uponreaction with primary and secondary amines. See H. Nakamura and Z.Tamura, Anal. Chem. 52, 2087 (1980); and D. Eastwood et al., Appl.Spectrosc. 60, 958 (2006). Following the reaction with fluorescamine thenanoparticles were washed thoroughly with hexane and methanol usingmagnetic separation to remove any unreacted fluorescamine. Thefluorescence of the nanoparticles was then measured using aspectrofluorometer with an excitation wavelength of 390 nm. An emissionpeak at 468 nm was observed, which is characteristic of the as-formedpyrrolinones, clearly indicating the presence of covalently bounddiamine on the surface of the nanoparticles. See M. G. Gore,Spectrophotometry and Spectrofluorimetry: A Practical Approach, 2nd ed.;Oxford University Press: New York, N.Y., USA. (2000).

Thermogravimetric analysis (TGA) along with differential scanningcalorimetry (DSC) were carried out to characterize the nanocomposite.These results are shown in FIG. 5. TGA showed that the nanocompositepossessed 62 wt. % of Fe/Fe_(x)O_(y) nanoparticles, significantly largerthan what has previously been reported for magnetic nanoparticles in anepoxy system. See J. Puig et al., J. Phys. Chem. C 116, 13421 (2012).This value is also significantly larger than has previously beenprepared by covalent linkage to a nanoparticle surface; Mochalinincorporated up to 14 wt % of nanodiamonds into a covalently linkedepoxy-amine network in a similar fashion. See V. N. Mochalin et al., AcsNano 5, 7494 (2011). A number of different features corresponding to thevarious components that make up the nanocomposite are observed in theTGA/DSC plots. At around 210° C., an endothermic peak in the DSC isobserved, which corresponds to boiling of 1,6-diaminohexane. TGA showsthis fraction accounts for 7 wt. % of the nanocomposite, where theslightly higher temperature indicates this is most likely uncrosslinkeddiamine that is bound to the nanoparticle surface. At 300° C., a strongendothermic peak is observed in the DSC, which can be attributed to thehigh temperature fully completing the curing of the epoxy network. SeeM. Kessler, Advanced Topics in Characterization of Composites, 1st ed.;Trafford Publishing: Bloomington, Ind., USA (2004). This is followed bya large mass loss observed in the TGA between 300° C. and 460° C., whichcan be attributed to the complete removal of the organic fraction (afurther 31 wt. %). From 460° C. to 620° C. a number of endothermicprocesses can be observed in the DSC which can be assigned to theevaporation of molecular oxygen from the oxide shell of thenanoparticles. The peak shape matches well with previous observations ofiron oxide reduction in alumina. See X. Gao et al., J. Chem. Soc.,Faraday Trans. 89, 1079 (1993). Here, the peak at 460° C., the shoulderon the peak at 600° C., and the peak at 600° C. correspond to thereduction profile; α-Fe₂O₃→Fe₃O₄→α-Fe. The peaks are shifted to slightlylower temperatures than previously reported due to the high curvature ofthe 15.2 nm nanoparticles.

The cured nanocomposite was characterized using VSM magnetometry.Saturation magnetization, M_(sat), of the nanocomposite cured using1,6-diaminohexane was measured to be 58 Am²/kg at 50 K. Initial masssusceptibility of the nanocomposite was observed to reduce from 1.0×10⁻³m³/kg for the Fe/Fe_(x)O_(y) core-shell nanoparticles to 5.2×10⁻⁴ m³/kgfor the cured nanocomposite. Using the known mass percentages from TGA,the saturation magnetization of the magnetic nanoparticles themselves inthe cured composite can be calculated, giving a value of 76 Am²/kg, asshown in FIG. 6(a). The drop in M_(sat) when compared to theas-synthesized nanoparticles indicates that there has been an increasein the magnetically dead layer at the surface. This is most likely dueto a further increase in the thickness of the oxide shell, althoughother factors may be present. Iron catalysts are known to be active inepoxide ring opening transformations, which may lead to irreversiblebinding at the surface of the nanoparticle and the introduction of amagnetically dead layer. See C. Bolm et al., Chem. Rev. 104, 6217(2004). While the ligand shell is expected to prevent access of theepoxide to the surface, the high lability of the 1,6-diaminohexane, dueto the relatively short 6 carbon chain, may allow some finite residencetime. To investigate whether the magnetically dead layer increasedduring nanocomposite formation, the M_(sat) curve was fit with theLangevin equation, as shown in FIG. 6(b). The magnetic diameter wascalculated to have decreased from 10.2 nm to 9.5 nm. See M. Unni et al.,ACS Nano 11, 2284 (2017). This indicates an increase in the thickness ofthe oxide shell from 2.5 nm to 2.9 nm, which also accounts for theoverall reduction in initial mass susceptibility as iron oxides possessa lower susceptibility than zero-valent iron.

The interparticle spacing can be tuned by changing the length of thediamine. To investigate the influence on interparticle spacing on themagnetic properties of the superparamagnetic nanocomposite,1,6-diaminohexane was substituted with longer alkyl chain diamines,namely 1,8-diaminooctane and 1,12-diaminododecane. Saturationmagnetizations, M_(sat), for the new nanocomposites were measured to be60 Am²/kg and 62 Am²/kg for 1,8-diaminooctane and 1,12-diaminododecane,respectively. These values are similar to those observed with1,6-diaminohexane, albeit slightly higher. The slight increase inM_(sat) may be due to the lower water solubility values observed as thealkyl chain length of the diamine increases, which reduces the extent ofoxidation during nanocomposite preparation.

Zero-field cooled experiments were performed to compare the onset ofsuperparamagnetism in the various nanocomposites, with the results shownin FIG. 7. The Fe/Fe_(x)O_(y) core-shell nanoparticles possessed ablocking temperature, T_(B)=233 K. In comparison, the as-curednanocomposites possessed blocking temperatures of T_(B)=147 K, 142 K,and 146 K for 1,6-dimainohexane, 1,8-diaminooctane, and1,12-diaminododecane, respectively. The reduction in T_(B) is consistentwith an increase in the size of the oxide shell on the nanoparticles.The lack of variation in the blocking curves indicates that increasingthe alkyl chain length from C6 to C12 still does not significantlychange the particle interaction behavior. In fact, this indicates thatan alkyl chain length as short as 6 carbons results in good separationof the magnetic fraction, as any magnetic interactions present in thesystem tends to drive T_(B) higher. See M. Kin et al., AIP Advances 6,125013 (2016). The well-defined blocking temperatures located below roomtemperature indicate that the nanocomposite is still superparamagnetic,and there is no evidence for the formation of ferromagnetic domains.

The present invention has been described as superparamagneticnanoparticles and nanocomposites. It will be understood that the abovedescription is merely illustrative of the applications of the principlesof the present invention, the scope of which is to be determined by theclaims viewed in light of the specification. Other variants andmodifications of the invention will be apparent to those of skill in theart.

We claim:
 1. A method to form a matrix-free superparamagneticnanocomposite, comprising: providing a plurality of superparamagneticnanoparticles having a surfactant on the surface of thesuperparamagnetic nanoparticles, exchanging the surfactant with an amineligand comprising at least two amine groups to provide aminatednanoparticles that express an amine functionality, reacting the aminatednanoparticles with an epoxy comprising at least two epoxide groups, andcuring the epoxy-reacted nanoparticles to provide a cross-linked epoxynetwork of covalent linkages between the superparamagneticnanoparticles.
 2. The method of claim 1, wherein the superparamagneticnanoparticles comprise iron, cobalt, nickel, or alloys thereof.
 3. Themethod of claim 1, wherein the superparamagnetic nanoparticles aresynthesized by a reversible agglomeration method.
 4. The method of claim1, wherein the amine ligand comprises an alkyl or aromatic aminecomprising two or more amine groups.
 5. The method of claim 1, whereinthe amine ligand comprises a diamine or triamine.
 6. The method of claim1, wherein the amine ligand comprises an alkyl diamine.
 7. The method ofclaim 6, wherein the alkyl diamine comprises diaminooctane ordiaminododecane.
 8. The method of claim 1, wherein the epoxy comprises atriepoxide.
 9. The method of claim 8, wherein the triepoxide comprisesN,N-diglycidyl-4-glycidyloxyaniline.